The present invention relates generally to an airport runway lighting system and more particularly to a bidirectional lighting system including means for switching illumination between the two directions. Still more particularly, the present invention comprises a selectable, bidirectional runway lighting system arranged for implementation in existing runway or new lighting circuitry.
Existing airport runways typically include a series of runway light fixtures secured along the centerline of an aircraft runway. In general, the fixtures direct light from a single lamp simultaneously along both approaches to the runway. The fixtures secured along a length of runway each are coupled by means of a series current transformer to a single alternating current transmission line extending from a constant current regulator through an electrical vault beneath the runway. The constant current regulator typically generates a current at one of five preselected amplitudes, including 2.8, 3.4, 4.1, 5.2, and 6.6 RMS amperes. A five-position manual intensity control switch located in the airport control tower connects via a multiconductor cable to the constant current regulator located in the electrical vault and controls the amplitude of current generated by the current regulator. The control switch thereby enables airport personnel in the control tower to select individually the intensity of runway light illumination for each runway.
It should be apparent to those having skill in the art that a runway lighting system that utilizes two lamps in each fixture, the illumination of each lamp being directed along a single runway approach, has certain advantages over a system using single-lamp fixtures. First, a two-lamp fixture with but one lamp illuminated at any one time will consume substantially less power to achieve a given illumination level than will a single-lamp fixture projecting the same illumination level along both approaches. Second, as a consequence of the lower power consumption in a two-lamp fixture and the nonuse of one lamp in each two-lamp fixture at any one time, the mean time between failure of a lamp in a two-lamp fixture will exceed that of a single-lamp fixture. Finally, use of a lighting system with two-lamp fixtures, which, unlike single-lamp systems, provides illumination along only one runway approach at any one time, confirms to aircraft pilots that they are approaching the runway from the proper direction.
Despite the inherent advantages of a runway lighting system using two-lamp fixtures, as one skilled in the art will appreciate, a switchable two-lamp system typically is prohibitively expensive to implement. Using technology well known in the prior art, such a system would require two separate current transmission lines along each runway instead of the single transmission line currently in use. In addition, each two-lamp fixture would require the use of two current transformers rather than one. Finally, an additional apparatus would be necessary in a two-lamp system to couple selectively one of the two current transmission lines to the single current regulator. Typically, the expense necessary to implement the foregoing lighting system using two-lamp fixtures exceeds any savings to be expected from such a system. Thus, prior art technology in the art of airport runway lighting systems does not provide a practical means for implementing a lighting system using two-lamp fixtures.